Lua sugar syntax double dots :

Using the sugar syntax : on Lua will allow us to access to the self table instance and for consecuent all methods and variables inside that specific class.

From Lua documentation we can read the following:

In the methods, we use a "self" parameter to get the instance to operate on. This is so common that Lua offers the : syntax sugar that calls a function entry from a table and inserts the table itself before the first arg.